Multicarrier demodulator

ABSTRACT

A device in which the frequency band (W) of a multiple carrier signal (MCS) is divided into a number K of sub-bands. The data contained in each sub-band are sampled at a predetermined rate (Sr) to generate a spaced sequence of samples. The data samples from all the sub-bands are time multiplexed to produce a composite signal comprising the K successive spaced sequences of samples. In this way, the composite signal transmits the data at a high apparent rate suitable for demodulation by a conventional multiple carrier demodulator designed for this high data transfer rate. This device is used in satellite communication systems.

The present invention relates to satellite data communication systemsand it is in particular concerned with an apparatus to demodulatesimultaneously a great number of data channels having low and mediumdata rates, that is data rates lower than 4 Mbit/s.

Nowadays the satellite communication networks are experiencing anevolution from the original trunk oriented network configuration inwhich the data are transmitted at a high data rate (typically 4 Mbit/s)towards user oriented network configurations including small stationnaryand mobile earth stations operating at relatively low data rates down toe.g. 64 kbit/s. In such networks, either private or public, the earthstations have different needs in communication capabilities inaccordance with the number of users which are to be served. The problemthen is to optimize the apportionment of the available communicationcapability between the different operating systems. Heretofore, sharingthe overall communication capacity was made by defining a range of datarates for each system. To cite two examples, the INTELSAT IBScommunication system operates at data rates from 64 kbit/s to 4 Mbit/s,while the EUTELSAT SMS communication system operates at data rates from64 kbit/s to 2 Mbit/s.

In order to process a great number of data channels it is known to usean apparatus called multicarrier demodulator (MCD) that is capable tosimultaneously process a great number of carriers modulated each by onedata channel. An apparatus of this type is disclosed for instance inU.S. Pat. No. 4,754,449. However, such a known apparatus is designed toprocess only data channels having a fixed data rate. Now in those newbusiness applications which are anticipated for satellite communicationsystems an earnest need has arisen for apparatus capable ofsimultaneously handling data channels having different data rates andmore particularly apparatus capable of simultaneously processing variouscombinations of data channels having various data rates without loss inbandwidth, for instance an apparatus which is capable of demodulating 16channels transmitting data at a rate of 2 Mbit/s as well as anyoperating combination up to 512 channels transmitting data at a rate of64 kbit/s still in the same frequency bandwidth.

A multicarrier demodulator capable of processing data having differentdata rates is disclosed in the publication Electrical Communication Vol.62, No. 3/4, 1988. This demodulator implements a signal processingmethod and comprises for each channel complex filters for translatingthe channel frequency band to the baseband, a synchronizer and a shapingfilter. The method which is carried out in this known demodulatorresults in an architecture that has the drawback of being complex andbulky, which makes it improper for being used on board a satellite.

The object of this invention is to remedy the above drawback byproviding a demodulator apparatus capable of processing data channelshaving variable data rates, which is simple to construct and less bulkyand which can thus be used either on board a satellite or in an earthstation.

In accordance with an aspect of the invention there is provided anapparatus for demodulating a multicarrier signal, comprising an inputcircuit as defined in the claims, by virtue of which a knownmulticarrier demodulator is made capable of processing data channelshaving different data rates.

The input circuit according to this invention is based on theimplementation of a method of processing a multicarrier signal occupyinga predetermined frequency bandwidth, in which, in accordance with afurther aspect of the invention, the frequency band of the multicarriersignal to be processed is divided into a predetermined number K ofsub-bands in each of which the number of modulated carriers is relatedto the data rate adopted for the respective sub-band. Then the datacontained in each sub-band or group of subbands are sampled at apredetermined rate to generate a plurality of expanded sample sequences.Said expanded sample sequences thus generated are thereafter timemultiplexed after one another thereby to produce a composite sequence inwhich the data have an apparent rate higher than the data rate in saidmulticarrier signal, the composite sequence being intended to bedemodulated in a conventional multicarrier demodulator.

The invention has the advantage of making it possible to use simple andless bulky demodulation circuitry, e.g. a conventional fixed parameterdemodulator intended to accept data having a high data rate in order todemodulate and reconstruct data having different lower data rates. Theinvention also makes it possible to reconfigure the general circuitarrangement of the demodulation circuitry in a simple and fast way sothat it is capable of processing data having different combinations ofdata rates without waste of frequency bandwidth. In short, the inventionaffords a great flexibility of operation and lends itself to a widerange of interesting applications.

The features and advantages of the invention will be more readilyapparent from the following disclosure when read in conjunction with theaccompanying drawings, in which :

FIG. 1 is a simplified schematic diagram illustrating the basicconfiguration of the arrangement according to the invention.

FIG. 2 illustrates an exemplary embodiment of the arrangement of FIG. 1.

FIGS. 3 and 4 show two examples of variations of the embodiment of FIG.2, especially adapted for two exemplary applications.

Referring to FIG. 1 which illustrates the basic configuration of thearrangement according to the invention, the reference numeral 20designates a multicarrier demodulator circuit, known per se, with thefollowing operation parameters: a number Nc of carriers and a nominaldata rate Dr. It is known that such a demodulator circuit is used tosimultaneously demodulate Nc carriers modulated by digital datadelivered at a data rate Dr. In accordance with this invention there isprovided an input circuit designated as a whole by reference numeral 10.This circuit, that shown connected between an input transmission line100 and the input of the demodulator circuit 20, has the purpose ofpermitting the demodulator circuit 20 to accept from the inputtransmission line 100 a multicarrier signal MCS containing severalcarriers modulated each by one digital data channel at a data rate lowerthan the nominal data rate Dr of the demodulator circuit 20.

Referring in particular to the circuit arrangement in block 10 in FIG.1, a set of K bandpass filters 11 are connected in parallel to receivethe multicarrier signal MCS occupying a frequency spectrum having abandwidth W. Each filter has a distinct pass bandwidth equal to W/KHertz such that the set of filters 11 is effective to divide thefrequency bandwidth W into a number of sub-bands, each of said sub-bandsbeing capable of transmitting Nc/K modulated carriers. At the output ofeach filter 11 there is connected a digitization circuit 12 thatoperates to sample the filtered signal at a predetermined rate andtranslates the signal back to the baseband thereby to produce a samplesequence on lines 101. The samples on each line 101 are received in astorage means 13 provided for buffering the samples. In the exemplaryembodiment illustrated, two buffers 13 and 14 are provided foralternately storing the sample sequences. While one of the buffers ineach pair of buffers is being loaded with a sample sequence, e.g. buffer13, the contents of the other buffer is read out by a time multiplexer15. The latter cyclically scans the outputs of all the buffers that arenot being loaded at this time, e.g. the buffers 14 in the operationstate as shown in FIG. 1.

With Sr designating the sampling rate, the time cycle of the timemultiplexer 15 is such that the contents of each storage means ispresented at one input 102 of the multiplexer at a rate equal to Sr*Kwhereby the frequency spectrum on line 102 is expanded to the bandwidthW of the input multicarrier signal MCS. The output of multiplexer 15produces a composite signal containing, after one another, the Kexpanded sample sequences that are sequentially read out of the storagemeans. It is to be noted that each sample is representing a time signalwhich contains the carriers applied at the input of a bandpass filter 11the frequency spectrum of which has been expanded to the bandwith W ofthe input multicarrier signal MCS and the data rate of which has beenexpanded to the nominal rate Dr.

FIG. 2 schematically represents an exemplary embodiment of the inputcircuit according to the invention arranged to feed a multicarrierdemodulator circuit 20, known per se, which is designed for eightcarriers (Nc=8) modulated by digital data having a data rate of 2 Mbit/s(Dr=2 Mbit/s). The input circuit 10 includes eight bandpass filters 11(K=8) operating to divide the frequency bandwidth W of the incomingsignal MCS in eight sub-bands. Each sub-band may contain eight modulatedcarriers whereby the incoming signal MCS in the same frequency bandwidthW, may contain 64 carriers that are modulated by digital data at a rateof 256 kbit/s, with the carriers having a frequency spacing equal to oneeighth of the nominal frequency spacing of the carriers when same aremodulated by digital data at a rate of 2 Mbit/s.

Each buffer 13 or 14 thus stores a sequence of data samples at 256kbit/s and the time cycle of the time multiplexer 15 has a durationequal to eight times the read time at the output of a buffer 14 or 13.At output 103 of the time multiplexer 15 there is thus produced acomposite sample sequence which comprises the eight expanded sequencesof samples successively read out of the eight buffers 14 or 13. Thecomposite sequence occupies a frequency spectrum having the bandwidth Wthat can be accepted by the demodulator circuit 20. The apparent rate ofthe digital data delivered by the time multiplexer 15 is then 2 Mbit/s,that is the data rate for which the demodulator circuit 20 was designedin the example described herein. It is therefore possible to use aconventional demodulator circuit in order to reconstruct the 8×8sequences of data at 256 kbit/s.

It is worthy of note that an existing multicarrier demodulator designedfor a predetermined data rate may be quite easily used, without anymodification of its design, for processing data channels having a lowerdata rate, subject to merely adding an input circuit in accordance withthe invention. Said circuit thereby makes it possible to broaden thefeasibility of using a multicarrier demodulator in satellitecommunication systems.

Basically, a circuit according to the invention achieves the resultsought for in terms of data rate of the digital data channels bycarrying out a method in which the frequency bandwidth of themulticarrier signal to be processed is divided into a predeterminednumber of sub-bands containing each a number of modulated carriers thatis chosen having regard to the data rate adopted for the respectivesub-band. The data contained in each sub-band or group of sub-bands aresampled at a predetermined rate thereby to generate a number of samplesequences the frequency spectrum of which is expanded to the fullbandwidth of the multicarrier signal. After buffering, the expandedsample sequences are multiplexed in the time domain after one anotherthereby to produce a composite sequence in which the data have anapparent data rate that is higher than the data rate in the incomingmulticarrier signal.

One noteworthy advantage of the invention resides in that it affords agreat flexibility of implementation by making it possible to processsimultaneously data channels which transmit digital data at differentdata rates provided that a proper bandpass filter configuration is used.As a matter of fact, the frequency subband transmitted by each filter 11may contain any number of carriers and convey a variable number of datachannels depending on the data rate adopted and the data in eachsub-band may be processed independently of the data contained in theother sub-bands. The data rate in each frequency sub-band may be chosenfreely between Dr and Dr/Nc, and the number of carriers transmitted byeach one of the bandpass filters 11 is equal to the ratio between thenominal data rate Dr and the real rate of the data modulating thecarriers.

Thus, a circuit according to the invention makes it feasible to transmitin one sub-band, for instance

2 carriers at a rate of 1 Mbit/s or

8 carriers at a rate of 256 kbit/s or

16 carriers at a rate of 128 kbit/s or

32 carriers at a rate of 64 kbit/s.

FIG. 3 illustrates an example of configuration arranged to handle datadelivered at three different rates: data at 1 Mbit/s in channels 1 to 4;data at 512 kbit/s in channels 5 and 6; and data at 256 kbit/s inchannels 7 and 8. In this exemplary embodiment the outputs from thebandpass filters 11 for channels 1 to 4 are added and the combined groupof the signals in these four channels is then sampled as a whole in acommon sampler 12 at a predetermined rate thereby to expand the samplesequence as generated to the bandwidth W of the multicarrier signal.Likewise, the outputs from the bandpass filters for channels 5 and 6 arecombined before sampling. As to the outputs from the filters forchannels 7 and 8, they are sampled separately as they are in thearrangement shown in FIG. 2. The expanded sample sequences, as bufferedin the storage means 13/14, are thereafter multiplexed as outlined inthe foregoing.

Considering generally an even number X of bandpass filters 11 among theK filters provided in the scheme, it is possible to sum the outputsignals from these X filters, to sample the resulting signal at asampling rate equal to Sr*X, and to couple these samples to a pair ofbuffers 13, 14 having a storage capacity equal to L*X. These two buffersserve the purpose of the X pairs of buffers 13 and 14 which would havebeen used in the basic configuration of FIG. 1.

The multiplexer 15 scans alternately the outputs of the storage meansand the time cycle of the multiplexer has then a duration equal to theread time at the output of one storage means multiplied by X. If Ncdenotes the number of carriers passing through the set of X filters andif the common data rate is (Dr/Nc)*X, then the resulting spectrum hassuch a bandwidth that the signal which is coupled to the input of themulticarrier demodulator conforms to the parameters Nc and Dr, and as aconsequence the signal is suited for being demodulated by a multicarrierdemodulator designed for parameters Nc and Dr. From the foregoing itwill be apparent that a great flexibility of operation results fromusing the invention.

The flexibility may still be extended further if the values of theparameters Nc and Dr are modified to Nc*K and Dr/K during the scanningcycle of the multiplexer. An example of such an application isillustrated in FIG. 4. In this example, two output processors 16, 17 areconnected to the output of multiplexer 15, said processors beingorganized to compute the fast Fourier transforms, one of said processorsbeing operating while the other one gets reprogrammed for processing thedata contained in the following frequency sub-band.

The circuitries for performing the various functions carried out in thearrangement according to the invention may obviously be implemented invarious embodiments that are within the ordinary skill of one skilled inthe art. It is also obvious that the bandpass filters implemented in thearrangement may be realized either in analog or digital form.

We claim:
 1. In an apparatus arranged to demodulate a multicarriersignal which occupies a predetermined frequency bandwidth and comprisesa plurality (Nc) of carriers, where Nc represents the number of carriersin said plurality, modulated by data transmitted having a firstpredetermined data rate, an input circuit for enabling a multicarrierdemodulator to process a plurality of data channels each having a datarate less than said first data rate, said input circuit comprising:firstmeans (11) connected to receive said predetermined frequency bandwidthand divide said frequency bandwidth into a number (K) of distinctfrequency sub-bands, where K represents the number of sub-bands eachfrequency sub-band being occupied by Nc/K carriers modulated by a datachannel having a data rate different from the first data rate, aplurality of sampling means (12) each being connected to the output ofat least one band dividing means, each sampling means being arranged tosample the output signal from said band dividing means at apredetermined rate so as to generate a distinct expanded samplesequence, in which a frequency spectrum is expanded to the bandwidth ofthe input multicarrier signal, a plurality of store means (13), each ofwhich being connected to the output of a sampling means so as to bufferthe expanded sample sequence as generated, multiplexer means (15)arranged to periodically scan each of said store means at apredetermined rate so as to read out successively each stored samplesequence and to produce a composite sequence containing the plurality ofsuccessive expanded sample sequences, said composite sequence beingapplied to the input of said multiplexer demodulator (20).
 2. An inputcircuit according to claim 1, wherein each sampling means (12) isassociated with a pair of store means (13, 14) arranged such that theinputs thereof are connected alternately to the output of said samplingmeans, the outputs of said pair of store means being connected to bescanned alternately by said multiplexer means (15), the output of one ofsaid pair of store means being provided for being scanned while theinput of the other of said pair of store means is connected to theoutput of the respective sampling means.
 3. An input circuit accordingto claim 1, wherein at least one of said band dividing means (11)accepts a subband occupied by a number of carriers different from thenumber of carriers in the other sub-bands, the carriers in said sub-bandbeing modulated by data having a rate different from the rate of thedata contained in the other sub-bands.
 4. An input circuit according toclaim 2, wherein at least one of said band dividing means (11) accepts asubband occupied by a number of carriers different from the number ofcarriers in the other sub-bands, the carriers in said sub-band beingmodulated by data having a rate different from the rate of the datacontained in the other sub-bands.
 5. An input circuit according to claim1, wherein the band dividing means (11) are comprised of analog bandpassfilters.
 6. An input circuit according to claim 1, wherein the banddividing means (11) are comprised of digital filters.
 7. An inputcircuit according to claim 1, further comprising control means (16, 17)connected to the output of the multiplexer means (15), said controlmeans being arranged so as to reconfigure the output of the inputcircuit to produce a predetermined data rate or rates.
 8. An inputcircuit according to claim 2, further comprising control means (16, 17)connected to the output of the multiplexer means (15), said controlmeans being arrange so as to reconfigure the output of the input circuitto produce a predetermined data rate or rates.
 9. An apparatus for usedwith a multicarrier demodulator arranged to demodulate a multicarriersignal occupying a predetermined frequency bandwidth and comprising aplurality carriers, where Nc represents the number carriers in saidplurality modulated by data transmitted at a first predetermined rate,thereby to enable the demodulator to demodulate a plurality of digitaldata channels having data rates less than said first data rate,comprising;first means (11) connected to receive said predeterminedfrequency bandwidth and divide said frequency bandwidth into a number(K), where K represents the number of sub-bands, of distinct frequencysub-bands, each frequency sub-band being occupied by Nc/K carriersmodulated by a data channel having a data rate different from the firstdata rate, a plurality of sampling means (12) each being connected tothe output of at least one band dividing means, each sampling meansbeing arranged to sample the output signal from said band dividing meansat a predetermined rate so as to generate a distinct expanded samplesequence. a plurality of store means (13), each of which being connectedto the output of a sampling means so as to buffer the generated expandedsample sequence, multiplexer means (15) arranged to periodically scaneach of said store means at a predetermined rate so as to read outsuccessively each stored sample sequence and to produce a compositesequence containing the plurality of successive expanded samplesequences, said composite sequence being intended to be applied to theinput of said multicarrier demodulator (20).
 10. An input circuitaccording to claim 9, wherein each sampling means (12) is associatedwith a pair of store means (13, 14) arranged such that the inputsthereof are connected alternately to the output of said sampling means,the outputs of said pair of store means being connected to be scannedalternately by said multiplexer means (15), the output of one of saidpair of store means being provided for being scanned while the input ofthe other of said pair of store means is connected to the output of therespective sampling means.
 11. An input circuit according to claim 9,wherein the band dividing means (11) are comprised of analog bandpassfilters.
 12. An input circuit according to claim 9, wherein the banddividing means (11) are comprised of digital filters.
 13. An inputcircuit according to claim 10, wherein the band dividing means (11) arecomprised of analog bandpass filters.
 14. An input circuit according toclaim 10, wherein the band dividing means (11) are comprised of digitalfilters.
 15. A method of processing a multicarrier signal occupying apredetermined frequency bandwidth and comprising a plurality of carriersmodulated by data having at least one predetermined data rate,comprising the steps of:(a) dividing the frequency bandwidth of themulticarrier signal to be processed into a predetermined number ofsubbands in each of which the number of modulated carriers is related toa ratio between a nominal data rate and the real rate of the datamodulating the carriers, (b) sampling the data contained in eachsub-band or group of sub-bands at a predetermined rate to generate aplurality of expanded sample sequences, (c) time multiplexing theexpanded sample sequences as generated after one another thereby toproduce a composite sequence in which the data have an apparent datarate greater than the data rate in said multicarrier signal.